A study in Finland checked the frequency of sauna usage against the rate of hypertension, or high blood pressure, by following more than 1,500 middle aged men for an average of 25 years. The New York Times wrote about it a couple days ago.

The study found that, when compared to subjects who used the sauna only once per week, those who used it 2 or 3 times were 24% less likely to have high blood pressure. Those who used it 4-7 times were 46% less likely.

"The warmth of the sauna...improves the flexibility of the blood vessels which eases blood flow, and the warmth and subsequent cooling down of a typical Finnish sauna induces a general relaxation that is helpful in moderating blood pressure. Also, sweating removes excess fluid, acting as a natural diuretic. Diuretics are among the oldest drugs used to treat hypertension." (NY Times)